The invention relates to a process for producing electrolytic capacitors with low equivalent series resistance, low residual current and high thermal stability, which consist of a solid electrolyte and an outer layer comprising conjugated polymers, to electrolytic capacitors produced by this process and to the use of such electrolytic capacitors.
A conventional solid electrolytic capacitor consists generally of a porous metal electrode, an oxide layer present on the metal surface, an electrically conductive solid which is introduced into the porous structure, an outer electrode (contact connection), for example a silver layer, and further electrical contacts and an encapsulation.
Examples of solid electrolytic capacitors are tantalum, aluminum, niobium and niobium oxide capacitors with charge transfer complexes, or manganese dioxide or polymer solid electrolytes. The use of porous bodies has the advantage that, owing to the high surface area, it is possible to achieve a very high capacitance density, i.e. a high electrical capacitance in a small space.
Owing to their high electrical conductivity, particularly suitable solid electrolytes are conjugated polymers. Conjugated polymers are also referred to as conductive polymers or as synthetic metals. They are gaining increasing economic significance since polymers have advantages over metals with regard to processibility, to weight and to the controlled adjustment of properties by chemical modification. Examples of known conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylenevinylenes), a particularly important and industrially utilized polythiophene being poly-3,4-(ethylene-1,2-dioxy)thiophene, often also referred to as poly(3,4-ethylenedioxythiophene), since it possesses, in its oxidized form, a very high conductivity and a high thermal stability.
Practical development in electronics is increasingly requiring solid electrolytic capacitors with very low equivalent series resistances (ESR). The reasons for this are, for example, falling logic voltages, a higher integration density and rising clock frequencies in integrated circuits. Moreover, a low ESR also lowers the power consumption, which is advantageous particularly for mobile, battery-operated applications. It is therefore desirable to reduce the ESR of solid electrolytic capacitors as far as possible.
European Patent EP-B-340 512 describes the production of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of the cationic polymer thereof, prepared by oxidative polymerization, as a solid electrolyte in electrolytic capacitors. Poly(3,4-ethylenedioxythiophene) as a replacement for manganese dioxide or for charge transfer complexes in solid electrolytic capacitors lowers the equivalent series resistance of the capacitor and improves the frequency behavior owing to the higher electrical conductivity.
In addition to a low ESR, modern solid electrolytic capacitors require a low residual current and a good stability with respect to external mechanical and thermal stresses. Especially during the production process, the encapsulation of the capacitor anodes involves high mechanical stresses which can greatly increase the residual current of the capacitor anode. When the capacitors are soldered on, high soldering temperatures of approx. 260° C. are used, which require a good thermal stability of the polymeric outer layer. The operation of the capacitors in an environment with elevated working temperature, for example in the automotive sector, also requires a high thermal stability of the polymeric outer layer.
Stability with respect to such stresses, and hence a low residual current, can be achieved in particular by an outer layer composed of conductive polymers with a thickness of approx. 5-50 μm on the capacitor anode. Such a layer serves as a mechanical buffer between the capacitor anode and the cathode-side contact connection. This prevents, for example, the silver layer (contact connection) from coming into direct contact with the dielectric or damaging it in the event of mechanical stress, thus increasing the residual current of the capacitor. The conductive polymeric outer layer itself should have so-called self-healing behavior: minor defects in the dielectric on the outer anode surface, which occur in spite of the buffer effect, are electrically insulated by virtue of the conductivity of the outer layer being destroyed by the electrical current at the defect site. The conductive polymeric outer layer must cover especially the edges and corners of the capacitor body, since the highest mechanical stresses occur thereon.
The formation of a thick polymeric outer layer by means of an in situ polymerization is very difficult. The layer formation requires very many coating cycles. As a result of the large number of coating cycles, the outer layer becomes very inhomogeneous; especially the edges of the capacitor anode are often covered insufficiently. Japanese Patent Application JP-A 2003-188052 states that homogeneous edge coverage requires a complicated balance of the process parameters. However, this makes the production process very prone to faults. In addition, the layer polymerized in situ generally has to be freed of residual salts by washing, which causes holes in the polymer layer.
An impervious electrically conductive outer layer with good edge coverage can be achieved by electrochemical polymerization. However, electrochemical polymerization requires that a conductive film is first deposited on the insulating oxide layer of the capacitor anode and this layer is then electrically contacted for each individual capacitor. This contact connection is very costly and inconvenient in mass production and can damage the oxide layer.
In European Patent Application EP-A-1524678, a polymeric outer layer is obtained by applying a dispersion comprising particles of a conductive polymer and a binder. With these processes, it is possible to obtain polymeric outer layers relatively easily.
However, the edge coverage in this process is not always reliable and reproducible. In addition, the thermal stability of the polymeric outer layer under prolonged stress at elevated temperature is insufficient.
European Patent Application EP-A-1746613 improves the process from EP-A-1524678 by virtue of solid particles having a diameter in the range from 0.7 to 20 μm being added to the dispersion. This significantly improves the edge and corner coverage. However, the addition of solid particles makes the polymeric outer film brittle, which can cause the outer layer to flake off locally and hence an increase in the residual current and in the ESR.
There was thus a need to improve the process for producing solid electrolytic capacitors described in EP-A-1524678 to the effect that better edge and corner coverage can be achieved without making the outer layer brittle.